Magnetic random access memory (MRAM) is a non-volatile thin-film memory that is used for data storage. A typical MRAM device includes an array of memory cells. Conductive traces (commonly referred to as word lines and bit lines) are routed across the array of memory cells. Word lines extend along rows of the memory cells, and bit lines extend along columns of the memory cells. Each memory cell is located at a cross point of a word line and a bit line, and stores a single bit of information.
The memory cells may be magnetic memory cells, such as spin dependent tunneling junctions. A typical magnetic memory cell includes a layer of ferromagnetic film in which the magnetization orientation is alterable (referred to as a sense layer or a data storage layer), and a layer of ferromagnetic film in which the magnetization orientation is fixed in a particular direction (referred to as a reference layer or a pinned layer). An insulating tunnel barrier is sandwiched between the ferromagnetic layers.
A logic value may be written to a magnetic memory cell by applying a magnetic field that sets the relative orientations of the memory cell's sense layer and reference layer to either parallel (logic “0”) or anti-parallel (logic “1”). The magnetization orientation in the sense layer aligns along an axis of the sense layer that is commonly referred to as its easy axis. External magnetic fields are applied to flip the magnetization orientation in the sense layer along its easy axis to either a parallel or anti-parallel orientation with respect to the magnetization orientation of the reference layer, depending on the desired logic state. The magnetization orientation of each memory cell will thus assume one of two stable orientations at any given time (i.e., parallel or anti-parallel). The parallel or anti-parallel orientation of the memory cell's ferromagnetic layers determines the resistance state of the memory cell, with a parallel orientation corresponding to a low resistance state, and an anti-parallel orientation corresponding to a high resistance state.
The external magnetic fields used to flip the magnetization orientation of the sense layer in a selected memory cell are created by supplying current to the word line and the bit line crossing the selected memory cell. The currents in the word line and bit line create magnetic fields that, when combined, can switch the magnetization orientation of the selected memory cell from parallel to anti-parallel or vice versa. Other unselected memory cells receive only a single magnetic field from either the word line or the bit line crossing the unselected memory cells. The magnitudes of the magnetic fields are chosen to be low enough so that the unselected memory cells do not switch their magnetization orientations when subjected to a single magnetic field from either the word line or the bit lines. An undesired switching of a memory cell that is subject only to the word line magnetic field or the write line magnetic field is commonly referred to as half-select switching.
As noted above, the logic value stored in a magnetic memory cell is determined by the parallel or anti-parallel orientation of the memory cell. Also, the parallel or anti-parallel orientation of the memory cell determines the resistance state of the memory cell. Thus, the logic value stored in the memory cell may be read by sensing the resistance state of the memory cell. However, the absolute difference between the resistance of a memory cell having a parallel orientation and the resistance of a memory cell having an anti-parallel orientation may be very small. Therefore, the act of measuring the resistance (i.e., reading the data in the memory cell) can itself introduce some uncertainty into the accuracy of the measurement. The act of measuring the resistance may also alter the magnetization orientation of the memory cell. If the magnetization orientation of the memory cell is altered (i.e., the reading operation is destructive), the data must also be written back into the memory cell after the data is read.